This invention relates to anti-G suits, of the kind used by pilots in high performance aircraft. A modern fighter aircraft is capable, as a structure, of withstanding the forces associated with carrying out manoeuvres up to about 15-G. However, even with the best of conventional precautions to protect the pilot, the pilot undergoes such physiological problems as blacking out at about 10-G. Under normal conditions, the pilot starts to suffer G-force-related problems at 6- or 7-G.
It is conventional practice to equip a fighter pilot with an anti-G suit, in which inflatable bladders are secured around the pilot's legs. A sensor senses the G-force on the aircraft, and inflates the bladders to an appropriate pressure. The pressurised bladders serve to prevent blood pooling in the pilot's feet, whereby the blood is prevented from draining from the brain, and causing blackout and the other physiological problems. Conventional anti-G suits add about 1-G to the pilot's tolerance of G-forces.
The designer of the anti-G suit is faced with the following considerations. A first problem is that, if the suit is inflated/deflated too slowly, the suit might actually harm the pilot, by trapping blood in the lower extremities.
A second problem is that the suit should not, for physiological reasons, be pre-inflated to a pressure higher than that dictated by the G-force, for more than a few moments, since that might seriously affect the pilot's blood pressure, and hence his capability/comfort level.
A third consideration is that it takes about half a second to one second, after the pilot has actuated the control stick to call for a change in G-force, for the G-force actually to come onto the aircraft. It takes the conventional anti-G suit and pressure regulation device about another half-second to a second to inflate the suit to the correct pressure as dictated by the new G-force on the aircraft. This speed of response is only acceptable if the aircraft changes G-force slowly, and if a second change in G-force is not commenced until the first change has been completed, and the suit inflated. Of course, in a fighter aircraft, the pilot may be changing his demands of the aircraft several times a second.
It may be noted that there are two components to the time it takes to effect a change in the pressure in the suit. First, the pressure regulator has to be re-set so as to admit a flow of air into, or out of, the suit; in particular, in the regulator, the size of the aperture or orifice through which air flows into or out of the suit has to be changed, and this change takes a measurable time. Secondly, there is the component of time that it takes the air actually to flow into or out of the suit. It is these two components in aggregate which take the second or half-second, as referred to.
With the conventional manner of controlling the inflation of the anti-G suit, it was not really worth reducing the suit-inflation time. If the suit inflation time were, at great expense, reduced below half a second, say, it would make little difference. For occasional changes of G-force (ie changes spaced more than about 2 seconds apart) the conventional pressure-control system could keep up. For more rapid changes in G-force, the suit pressure could not begin to change until after the G-force was established, which swamped any such minor gain in the suit-inflation-time.
It is known, however, that it is possible to predict the G-force that will be present in the aircraft in about one second's time. That is to say: although it takes about a second after the pilot actuates the control stick for the G-force to actually come onto the aircraft, it is possible to compute, more or less immediately after he has actuated the control stick, what that G-force will be. Therefore, it is possible, at least theoretically, to set the pressure in the suit according to that predicted G-force, rather than to the actual G-force already established on the aircraft. If this is done, the second or half-second that is used up in inflating the suit can run concurrently with the second it takes for the G-force to build up. Thus, the suit can be inflated, ready, by the time the G-force comes on.
The system of adjusting the suit pressure not to the actual G-force on the aircraft, but to the G-force that will be appearing on the aircraft in one second's time, is called the Preview Control system. The system yields marked advantages over the conventional system, because inflation of the suit can be initiated a sufficient time before the G-force comes on for the suit to be inflated to the correct pressure to support that G-force. Improvements in the pilot's tolerance are predicted to be as high as 2- or 3-G extra, especially in cases where the pilot is requiring rapid changes of the G-force.
As mentioned, with the conventional suit-pressure control systems, there was little to be gained by reducing the time taken to actually inflate the suit. By contrast, with Preview Control, any saving in the time it takes to inflate the suit, instead of being just of marginal interest, will now be very useful. When the G-force on an aircraft changes, it does not change according to a smooth linear ramp function, but rather in a more complex fashion: slowly at first as the control surfaces on the wings etc are adjusted; then rapidly; then slowly again as the new G-force is approached. If the suit can be pre-inflated in less than one second, there will be an improvement in the accuracy with which the suit pressure will be able to follow this pattern of change of the G-force.
It is emphasised that there is nothing that can be done, outside of a total re-design of the whole concept of fighter aircraft, about reducing the up to one second it takes from the time the pilot actuates the control stick to the time the G-force comes on. (This period is not stated as a fixed constant: of course, the greater the desired change in G-force, the longer the period will be. The period of up to one second is stated as being typical of the time it takes, from the moment the pilot starts to actuate the control stick, in practice and in a real aircraft, for a substantial change in G-force to become established in the aircraft.)
However, it is recognised that, with development work, there is something that can be done about redesigning the suit and its inflation system, to reduce the time it takes to inflate the suit. But the key step forward of Preview Control is that the suit starts to become inflated, not after the G-force has become established in the aircraft, but a half second or up to one second earlier than that, ie as soon as the pilot actuates the stick.
The present invention is concerned with combining a pressure regulation system into a Preview Control suit inflation system, which will permit the pressure in the suit to be changed, accurately, and with stability, in appreciably less time than the half to one second that such inflation has conventionally taken.
A number of approaches to the pressure regulation requirement will now be discussed.
First, it should be noted that the pressure regulation requirement is a most demanding one. The time it takes to change the G-force, and therefore the maximum time that can be allowed to change the pressure in the suit, if Preview Control is to work at all, is one second. If the suit pressure can be changed faster than that, so much the better: the time saving can be used, not to change the suit pressure too soon ahead of time, but to make the changes in the suit pressure follow the changes in the G-force more accurately.
The nature of the aircraft and its relation to the suit should be borne in mind in this context: consider, for example, if the suit, in order to perform its function, had to change its pressure very quickly (say in one-fiftieth of a second) no pressure regulation system could ever keep up, and high-performance anti-G suits could never be established. If, on the other hand, the aircraft were such that five seconds were available for changing the suit pressure, it would be so easy then for the pressure regulation system to keep up that virtually any type of regulator would serve.
It is recognised, as an aspect of the invention, that the time taken to change the pressure in the suit can be reduced down below half a second, which is very worthwhile, or even less, by the arrangements as described herein. It is recognised that this order of magnitude of a reduction is exactly what is required to convert Preview Control of anti-G suit inflation from a theoretical desideratum into a practical reality.
Again, it is pointed out that the time taken to effect a change in the pressure in the suit is in two components: the time to change the aperture opening in the regulator, plus the time to transfer the air into, or out of, the suit. In Preview Control, a computer on the aircraft is supplied with signals not only from the pilot's actuation of the control stick, but also from sensors which indicate, among other things, the aircraft's present G-force, the cockpit pressure, and such factors as the aircraft's altitude, speed, weight, and many other factors that affect the prediction of the G-force. The computer calculates the ideal pressure in the suit needed for the pilot to tolerate that G-force.
The invention provides a comparator, which makes a comparison between this ideal pressure that will be required in one second's time, and the magnitude of the pressure as it now is, as derived from a pressure sensor in the suit, and calculates the rate at which air must be admitted into the suit (or released from it) in order for the suit pressure to be at that ideal value when that G-force comes on.
The comparator computes how much inflation-air, or rather how great a flow rate of inflation-air, should be admitted into the suit. The pressure regulator therefore must be of the type that is able to receive a signal from the comparator, and to respond to such signal by automatically opening the aperture in the regulator to the appropriate size of opening.
One type of pressure regulator that might be considered for use with Preview Control is the type based on a solenoid-operated on/off valve. With such a regulator, the computer is programmed to energise the solenoid when the valve should be open and de-energise it when the valve should be closed. This simple type of pressure regulator is adequate, however, only when the changes in G-force are well-spaced apart (eg more than two seconds apart). This type of regulator may be termed the solenoid on/off regulator.
Suppose, as an example, that tests have shown that the ideal pressure for a particular G-force in a particular suit should 6 psi. Now, in order to inflate the suit from, say, 3 psi to 6 psi, it is necessary to turn the regulator off at about 5 psi, because otherwise, if left open, the pressure would be still rising when it reached 6 psi, and the pressure in the suit would overshoot. Turning the incoming pressure off at 5 psi so that the pressure just settles to 6 psi is something that can be programmed into the computer. If the change is from 5 psi to 6 psi, equally, the cut-off at 5.8 psi, or as required, again can be programmed into the computer. But these values are empirical, and depend on the computer "knowing" both the start pressure and the final or aimed-for pressure. Therefore, if the aimed-for pressure should be changed before it has been reached, ie by the pilot having again actuated the control stick, then the computer/regulator combination cannot possibly keep up, and the pressure in the suit will be awry, perhaps wildly so. With an on/off-based pressure-controller, the more the pilot requires a change in G-force before an earlier change in G-force has been completed, the more unpredictable the action of the regulator will become. The simple on/off solenoid type of pressure regulator therefore depends on the computer starting from a situation where the G-force and the pressure were in synchronisation, that is to say, the pressure in the suit was at the ideal pressure as required for that G-force. The computer cannot be allowed to start a new computation when the "start" pressure for that computation pressure has not yet reached correspondence with the G-force. Therefore, the simple solenoid type of pressure regulator cannot be used when the pilot requires rapid changes in the G-force.
Another conventional type of pressure regulator includes a spring that acts on a valve member. The pressure in the suit also acts on the valve member, so that, if the pressure in the suit is too low, the spring drives the valve member open, admitting more pressure into the suit. As the suit pressure increases, the valve gradually closes, thereby reducing the flow rate into the suit as the ideal pressure approaches. This type of regulator therefore has a built-in protection against overshoot: that is to say, the valve aperture becomes smaller as the pressure differential becomes smaller.
This type of regulator may be termed a balance-spring-against-pressure type of regulator.
If this type of regulator were selected for use with the anti-G suit, the size of the opening through which pressurised air passes to the suit would not be controlled, or not controlled directly, by the computer. The computer would control the force on the spring, and the size of the opening would be determined by the force on the spring as balanced by the pressure in the suit.
In order to provide that the opening is large when the difference between the spring and the suit pressure is large, the designer would be constrained to give the spring a low stiffness rate; ie the designer provides that the spring moves a substantial distance for each unit of the difference. But if the spring is of a low stiffness rate, the spring is floppy, and the regulator is thus able to hunt and overshoot and be otherwise unstable.
On the other hand, if the designer of a balance-spring-against-pressure type of regulator makes the spring rate too stiff, then the size of the opening will not change much for a given unit of difference, and in that case the regulator will lack the sensitivity required for it to open wide when the difference is large and open only a little when the difference is small.
If he were to use the balance-spring-against-pressure type of regulator, therefore, the designer would be forced to compromise between a high spring stiffness rate that gives too little sensitivity, and a low spring stiffness rate that promotes instability. It is recognised that this compromise cannot be met with a balance-spring-against-pressure type of regulator, when it comes to pressurising a practical anti-G suit in a practical aircraft, when the pilot is calling for the G-force to change for a second time before a first change has been completed. So, in the cases of both the solenoid on/off type of regulator and the balance-spring-against-pressure type of regulator, as just described, the regulators simply cannot be made to keep the suit correctly inflated to the correct pressure, when the pilot is calling for rapid changes in the G-force on the aircraft. It is not a question of increasing the size of the pipes and valves: even with the components of optimum sizing, the compromise between sensitivity and controllability cannot be met.
An aircraft anti-G suit has pockets which have a total capacity, typically, in the five to ten liters range. The suit is pressurised to a maximum of about ten or twelve psi. The material of the pockets is resilient to a certain degree, so that the volume of the suit increases as it is pressurised. With these parameters, changing the pressure from 3 psi to 6 psi in half a second, accurately, and with stability, is a most demanding task. The task is made doubly difficult if the required or aimed-for pressure should be changed before the initial aimed-for pressure has been reached. It is recognised that conventional pressure-regulation systems are not equal to the task.
It is recognised that Preview Control of the pressure of an anti-G suit however can be made to work, provided the suit pressure is controlled by the pressure regulation system as described herein. The benefits of Preview Control, provided the pressure can be properly regulated, can be expected, it is aimed, to be that the maximum G-force the pilot can withstand can be as high as 10-G. Even more important than the maximum G-force is the fact that the pilot can withstand rapid changes in G-force, and can withstand rapid rates of change of the G-force.
In the invention, the pressure regulator has a fluid-flow aperture connecting a pressure source with the suit, in which the aperture is of variable size. The regulator is such that the valve member of the regulator can be held at the partially open position.
In the invention, the size of the aperture is changed very rapidly, and yet with stability. One preferred manner in which the rapidity-stability requirement might, in the context of the invention, be numerically defined, is proposed as follows.
In the preferred definition, the time taken to effect a stable change in the size of the aperture is the time taken from the moment of initiation of the size change until the moment the change is stabilised to the new opening size, and is stabilised thereto with a deviation error from the required new size of less than one tenth of the magnitude of the change.
In the invention, it is preferred that the time taken to effect such a stable change is so rapid that the time interval between these two moments is less than about 100 milliseconds.
It is recognised that one of the keys to making Preview Control work is to provide a controlled force to change the size of the aperture opening in the pressure regulator very quickly. There are limits to what can be done to reduce the actual air flow rates into and out of the suit pockets, since the pockets are a little elastic, and there is a limit to how fast air can move through passages and pipes, whereby the inflation time of the suit, from opening the aperture to the suit being inflated (and the aperture reduced to zero) is quite long: of the order of 300 or 500 milliseconds. The invention lies in getting the aperture valve-member to change its size in a controlled and stable manner in a much shorter time interval than that.
The invention lies in recognising that a progressively-opening valve member (ie the member that opens/closes the regulator aperture), and a powered or active servo-system to power the valve member between openings, will provide the required degree of rapid yet stable response. A passive system for opening the valve-member, where the opening force comes from the pressure itself (as in the conventional spring/piston regulators) cannot have the quick, stable response required. A designer can vary the size of the opening by providing a bank of on/off valves, and switching in more or less of the valves. But on/off valves cannot be cycled on/off rapidly, ie slamming rapidly from fully open to fully closed, and expect to have a long service life.
It is recognised that the valve member can be made to move to the new opening size of the aperture rapidly if the valve member opens the aperture progressively, and if a powered servo-system is provided, which powers the valve member to the new opening size.
The invention reduces the time it takes for the pressure regulator to effect a rapid but stable change in the size of the aperture. As a result, it now becomes very worthwhile also to reduce the time taken for the air to flow into and out of the suit. Once the invention is in place, careful attention to pipe sizes, etc, can be expected to pay off in sharper control of the suit pressure.
The reduction in the time taken to effect a stable change in the aperture size means that the changes in suit pressure can be accomplished in markedly less time than it takes for the G-force actually to come on to the aircraft. Advantage can be taken of this reduction in time to make the suit pressure conform not just to the changes in G-force, but to different rates of change of the G-force. It may be that the ideal suit is one that exactly follows the changes in G-force as they come on to the aircraft, or it may be that having the suit-pressure slightly anticipating the upcoming G-force gives better pilot performance results. The point is, the invention permits either to be tried: the prior art pressure regulators were so unresponsive that it was hardly possible even to experiment with such determinations.
Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: